How to Minimize Performance Issues When Coating Hot Melt PSAs

Two main factors can lead to degradation and impact end-use performance once you are coating HMPSAs in your processes:

• Heat exposure: Based on your specific processing system, it is possible more heat is applied than necessary during production, which can lead to changes in the adhesive’s structure such as reduction in molecular weight. Further, if the adhesive experiences overheating, its polymer chains can break down. Both scenarios may cause thermal degradation and decrease the formulation of bonding properties that enable your desired performance levels.
• Oxidation: Again, based on your specific processing system, oxygen exposure at elevated temperatures for extensive periods of time can alter adhesive properties, including causing discoloration or char formation. Oxidation can also be accelerated if the adhesive is exposed to air at elevated temperatures, as that further weakens the polymer structure. 

Both heat exposure and oxidation can change the adhesive’s elasticity and viscosity levels, altering how it meets dispensing requirements and spreads across a substrate at the desired rate of flow. This, in turn, can impact the following:

• Coat weight: When heat and oxidation reduce polymer molecular weight, the overall viscosity can increase or decrease, especially under shear stress. This might mean you need to pump less despite your current equipment settings, and therefore may cause unexpected changes in coat weight and performance.
• Wettability: Changes in viscosity due to degradation can impact the wetting behavior, changing the bonding compatibilities with your desired substrates. If the adhesive’s ability is compromised, it may cause inconsistencies in coating surfaces evenly, which may also impact the ease of application in addition to ensuring proper bond strength throughout.

Polymers in HMPSAs are considered the “backbone” of the adhesive, as they provide internal strength to the HMPSA by improving elasticity and increasing viscosity. When breakdown occurs from HMPSA degradation, internal cohesive strength and the polymer’s ability to hold molecules together can be altered. While these changes may be positive or negative, it can lead to performance results you may not be expecting for your end-use application.

Changes with Internal Cohesive Strength

An HMPSA’s internal strength always consists of a balance between adhesive flow, wet out and bonding according to your application needs. When the polymer breakdown takes place, these components can become unbalanced. It’s possible the HMPSA may still be reusable if heated and cooled, but it could result in unexpected performance changes, such as reduced strength. However, if the HMPSA is heated for too long or exposed to too much oxygen, it will not be reusable at all. If changes occur with the balance of internal strength, it can also impact:

• Shear: If cohesive strength lessens, the HMPSA may become more susceptible to shear-induced deformation or failure because the adhesive may no longer be capable of withstanding external forces. This may shorten the end-use application’s lifespan, especially for durable labels or films that require strong shear.
• Tack levels: While tack typically enables HMPSAs to adhere to difficult substrates, an imbalance of internal strength may affect this capability. This is because an altered molecular structure will not be able to properly form and maintain its bond (i.e., its tack) with minimal pressure. This can cause inconsistent bonding strength amongst products or decrease overall performance for applications that require strong bonding to difficult substrates such as corrugate.

As the adhesive solutions segment of Arkema, Bostik’s technical experts are here to help you address performance issues that may arise once you are in the coating process. Our HMPSAs are designed to address your specific application needs with the ability to overcome degradation-related issues that may occur.

Bostik’s HMPSA Formulations

Designed to provide a balance of adhesive flow, wet out and bonding capabilities, the polymers in Bostik’s HMPSAs offer the necessary elasticity and viscosity levels for your end-use application needs. For example:

• Tackifying resins in our formulations help strengthen the bond between two substrates and ensure optimal tack balance for desired peel adhesion performance. They also provide softening and plasticizing effects to enhance overall flexibility.
• Plasticizer additives can also help enhance overall flexibility and maintain a certain amount of flow. Further, they can improve thermal stability and increase wet out capabilities.

Additionally, Bostik can help minimize oxidation exposure within the initial formulation to reduce possibilities of changes in internal strength during the coating process. In particular, this entails incorporating:

• Antioxidants, which slow down oxidation processes by preserving the molecular structure of the polymer, helping maintain cohesive strength. They also mitigate thermal stability when the adhesive is enduring high temperatures, protecting against heat-induced degradation. This ability to slow down oxidation and enhance thermal stability helps extend the overall shelf life of HMPSAs to streamline supply costs.
• UV stabilizers, which are designed to protect polymers from ultraviolet radiation, as it can cause changes to molecular structure and reduce cohesive strength. By absorbing or dissipating UV energy, UV stabilizers prevent any negative impacts to the HMPSA. This is particularly useful for outdoor applications that may ensure sun exposure or temperature changes.

Additionally, our HMPSAs help you reduce environmental impact. Comprised of 100% solids, they are low VOC formulations that offer fast set-up times to create a streamlined, safe work environment. Further, our formulations are compatible with bio-based raw materials to decrease carbon emissions.